The present invention relates to a system and method for tracking and correcting, in real-time, a motion of an anatomical object. In particular, the system and method uses octant navigators to track and correct certain features of the anatomical object (e.g., a brain) during a three dimensional scan thereof.
Several approaches have been utilized to correct for a movement of a subject during scans. Such conventional methods require an off-line, post-processing of the images. Indeed, an image correction is not theoretically possible in some cases because certain areas of k-space may not be sampled, while others are oversampled. The earliest navigator-based methods for the motion correction utilized straight-line navigators to detect a linear motion. Such technique may be useful in chest examinations where the diaphragm and associated organs translate along a particular axis. However, these conventional methods do not quantify and determine the magnitudes or degrees of rotations of the objects being examined, or portions thereof.
Other conventional systems and methods use correlated volumes to calculate rotations and translations. While this technique allows for a correction of certain types of motions, it cannot be applied to three dimensional single image structural scans. Indeed, the techniques employed by these systems and methods are computationally intensive. Similar approaches have been used on-line and off-line for two dimensional sequences. In particular, slices can be correlated to calculate the transformation from one slice to the next, and correct for such changes. However, while this conventional method has certain benefits, the correction of the motion provided therein only applies to the object motion for an in-plane movement.
Another conventional approach for the motion correction utilizes a set of navigator pulses. For example, circular navigators have been proposed to gauge a rotation within sequences. The circular navigators are 30 ms in duration, and three of these circular navigators are required to characterize the motion of the object about three cardinal axes. An iterative approach is then taken to correct the motion of the object since the navigators are not usable for the out of plane rotations. Therefore, this conventional method provides an approximate correction. Another set of three circular navigators is then acquired for a final (fine) adjustment. While this approach fulfills the theoretical need to compensate for all three axes of motions, there are certain impracticalities associated with such conventional approach. Most importantly, the entire procedure is relatively time-consuming, i.e., adding this procedure to, e.g., a standard functional magnetic resonance imaging (xe2x80x9cfMRIxe2x80x9d) acquisition sequence is not practical.
It is therefore the object of the present invention to use navigator echoes during each radio frequency acquisition to track the motion of an object (e.g., the head of a patient) and to compensate for such motion by correcting the gradients during the scan thereof. Another object of the present invention is to utilize the navigators to track and correct for a motion of the object in real-time. Yet another object of the present invention is to provide an improvement in the basic signals to measure the brain physiologic changes that are being investigated. Another object of the present invention is to improve the quality of anatomical images obtained. Still another object of the present invention is to assess (and possibly measure and record) the rigid-body motion (e.g., rotations and translations in 3 dimensions) without necessarily correcting for it.
These and other objects of the present invention are achieved with a method and system for correcting for a motion of an object. With these system and method, the navigator data and map data are obtained for the object. Then, the navigator data is compared to the map data to generate comparison data. A translation and/or a rotation of the object is corrected, in real time, as a function of the comparison data. The navigator can be preferably an octant navigator.
In one exemplary embodiment of the present invention, a scanning sequence can be used to determine a position of the object. This scanning sequence may include a signal portion which includes at least one radio frequency signal, an octant navigator portion which includes at least one octant navigator, and a spoiler portion provided for reducing a signal magnitude of the scanning sequence. The octant navigator is provided for allowing a measurement of at least one of a rotation and a translation of the object. The navigator portion is advantageously provided between the signal portion and the spoiler portion.
In another advantageous embodiment of the present invention, the octant navigator is a navigator that traces the outline of an octant on the surface of a sphere in k-space. This enables a rotation about the three cardinal axes, and a translation in all three directions to be achieved in a single read after a single radio frequency pulse.
It is also advantageous, according to the present invention, to first obtain a pre-mapping of the region of k-space a small number of degrees in each direction from the initial octant navigator. This pre-mapping technique preferably eliminates the need for an iterative, approximate solution. By comparing the actual navigator with a local pre-mapped k-space map, it is possible to determine the true rotations and translations using a single subsequent octant navigator. It is preferable that the rotations are not so large as to shift the navigator outside the boundaries of the map. Since the gradients can be preferably corrected after each navigator is acquired, the accumulated rotation may exceed the limits of the map, provided that the inter-navigator rotation is constrained by the limits of the map.
In still another embodiment of the present invention, the pre-mapping technique may be combined with an optimal radius selection method using concentric octant navigators. This procedure selects the optimal radius for an individual object to be used for the octant navigators in the map, and along with the subsequent navigators. Also, the optimal offset angles relative to each of the three axes may be selected. The octant navigators can also be inserted between the phase steps of a three-dimensional sequence to obtain a single motion corrected structural scan. They can also be inserted within and/or between the volumes of a three dimensional echo planar image used for functional imaging or diffusion imaging. Further, a slab selection may be used to eliminate parts that do not move as a rigid body with the part of interest.
According to an exemplary embodiment of the present invention, the path of the octant navigant does not have to be the edge of an octant. In fact any path on a sphere in k-space may be traced. Indeed, any path that extends in all three dimensions (i.e., preferably not a path that lies completely within a single plane) can be utilized. It is within the scope of the present invention to extract the navigator information. While all paths do not have to be equal or yield the same SNR in the rotation/translation estimates, the use of the octant is preferable. According to the present invention, a preliminary exploration and analysis of the k-space representation of a volume can yield an optimal path. It is possible, with an appropriate spoiling, to implement this exemplary scheme in two dimensional sequences. In this case, more than one map may be collected to gauge the effect on the navigators of the excitation of the different slices due to the imaging part of the sequence.
One of the exemplary advantages of the present invention is that it utilizes a navigator that is capable of reading in a single path (which may use only one radio frequency (xe2x80x9cRFxe2x80x9d) pulse that may be shared with the imaging part of the sequence. It is also within the scope of the present invention not to require an additional RF pulse for the navigant which can fully qualify a rigid body motion (rotations/translations) for which the navigant should occupy 3 dimensions, i.e. preferably not contained within a plane. The navigator used by the present invention can be applicable in 2 dimensional and 3 dimensional sequences for motion correction.